LIGHT DIFFUSION IN THE TROPICAL DRY FOREST OF COSTA RICA
S. Calvo-Rodriguez
a,
, G.A Sanchez-Azofeifa
a a
Earth and Atmospheric Sciences Department, University of Alberta, Edmonton, AB, Canada, T6G 2E3 - calvorod, gasancheualberta.ca
Commission WG VIII7 KEY WORDS: Leaf area index, spectral vegetation indices, MODIS
ABSTRACT: Leaf Area Index LAI has been defined as the total leaf area one-sided in relation to the ground. LAI has an impact on tree growth
and recruitment through the interception of light, which in turn affects primary productivity. Even though many instruments exist for estimating LAI from ground, they are often laborious and costly to run continuously. Measurements of LAI from the field using
traditional sensors e.g., LAI-2000 require multiple visits to the field under very specific sky conditions, making them unsuitable to operate in inaccessible areas and forests with dense vegetation, as well as areas where persistent sunny conditions are the norm like
tropical dry forests. With this context, we proposed a methodology to characterize light diffusion based on NDVI and LAI measurements taken from the field in two successional stages in the tropical dry forest of Santa Rosa National Park in Costa Rica.
We estimate a K coefficient to characterize light diffusion by the canopy, based on field NDVI measurements derived from optical phenology instruments and MODIS NDVI. From the coefficients determined, we estimated LAI values and compared them with
ground measurements of LAI. In both successional stages ground measurements of LAI had no significant difference to the tower- derived LAI and the estimated LAI from MODIS NDVI.
Corresponding author
1. INTRODUCTION
Leaf Area Index LAI plays a key role on light interception by the canopy, as the leaf area attenuates and reduces the
transmission of radiation to the forest interior. In many forests with closed canopies, only a small fraction 0.5-5 of the solar
radiation incident above the canopy reaches the understory Chazdon and Pearcy, 1991. In Tropical Dry Forests TDF
around 6-22 of the solar radiation incident above the canopy reaches the understory Lebrija-Trejos et al. 2011 because of
the low canopy height, simple vertical stratification and low leaf biomass.
TDFs are defined as a vegetation type dominated by deciduous trees at least 50 of the trees are deciduous, with an annual
average temperature of at least 25°C or higher, annual precipitation of 700-2000 mm per year, and a dry season
precipitation less than 100 mm of three or more months Sánchez-Azofeifa et al. 2014. In TDFs LAI varies seasonally,
having a maximum value during the growing season when water is available and a minimum value at the end of the dry season
Mass et al. 1995; Kalacska et al. 2005. These forests are characterized for a fast increase from LAI=0 in the dry season to
full canopy coverage LAI higher than 7 within a few days after the first rains, and a slow but sustain loss of leaves at the
end of the raining season until it reaches again a value of 0.
Although LAI is important to characterize ecosystem process, availability of LAI data is rare for TDFs Kalacska et al. 2005.
Moreover, a systematic bias exists for LAI observations since most of the data is collected in old growth tropical rain forests
without considering differences in canopy structure and composition found in secondary forests or the differences that
may exist with other ecosystems Murphy and Lugo, 1986; Weaver and Murphy, 1990; Lean and Rowntree, 1993;
Kalacska et al. 2005. Although many instruments exist to characterize LAI from the
ground, these methods are often laborious and costly. Unlike most flux sensors that are designed to run in all weather
conditions, most ground-based instruments to estimate LAI operate largely under conditions of no precipitation Wilson and
Meyers, 2007 and their observations are punctual in nature since they are related to specific field campaigns when data is
collected
once without
proper temporal
follow-up. Measurements of LAI using traditional optical sensors e.g.,
LAI-2000 also require multiple visits to the field under very specific sky conditions, making them unsuitable for inaccessible
areas and forests with dense vegetation, as well as in areas where persistent sunny conditions are the norm.
Continuous estimations of LAI using remote sensors can be obtained as a function of the spectral Vegetation Indices VIs,
such as the Normalized Differential Vegetation Index NDVI Wilson and Meyers, 2007. NDVI is currently the most widely
used reflectance vegetation index Pontailler et al. 2003. NDVI can be a sensitive indicator of the amount and vigor of
the vegetation, because the two wavebands used to estimate it represent the section of the solar spectrum on which the
Photosynthetically Active Radiation region takes place PAR, 400nm to 700nm Carlson et al. 1994.
In this context, the main objective of this study was to characterize light diffusion through the canopy in a TDF
ecosystem at two different levels of ecological succession by integrating optical phenology observations. A successional
stage is defined here as phase on which a given forest is found since its processes of functional recovery started as a
community. Depending of their level of succession, a forest
This contribution has been peer-reviewed. doi:10.5194isprsarchives-XLI-B8-579-2016
579
canopy will have different vertical and horizontal structures, as well as different levels of species, functional traits Quesada et
al. 2009 and light attenuation.
2. METHODS 2.1. Study area
